Accurate position information of user equipment (UE) such as cellular telephones, personal communication system (PCS) devices, and other mobile stations (MSs) is becoming prevalent in the communications industry. The Global Positioning System (GPS) offers an approach to providing wireless UE position determination. GPS employs satellite vehicles (SVs) in orbit around the earth. A GPS user can derive precise navigation information including three-dimensional position, velocity and time of day through information gained from the SVs.
GPS systems determine position based on the measurement of the times of arrival at a GPS receiver antenna of the GPS signals broadcast from the orbiting SVs. Normally, reception of signals from four SVs is required for precise position determination in four dimensions (latitude, longitude, altitude, and time). The observed signal propagation delay is the difference between the observed signal transmit time and the assumed local receive time. A pseudorange is constructed by scaling the observed propagation delay by the speed of light. The location and time are found by solving a set of four equations with four unknowns incorporating the measured pseudoranges and the known locations of the SVs. The precise capabilities of the GPS system are maintained using on-board atomic clocks for each SV, in conjunction with tracking stations that continuously monitor and correct SV clock and orbit parameters.
One disadvantage of the GPS system for location determination is the relatively long time needed to perform signal acquisition under certain conditions. SV signals cannot be tracked until they have first been located by searching in a two-dimensional search “space”, whose dimensions are code-phase delay and observed Doppler frequency shift. Typically, if there is no prior knowledge of a signal's location within this search space, as would be the case after a receiver “cold start”, a large number of code delays and frequencies must be searched for each SV signal that is to be acquired and tracked. These locations are examined sequentially, a process that can take several minutes in a conventional GPS receiver.
GPS receivers must acquire signals from SVs whenever the receiver has lost reception, such as, after power down, or when the signal has been blocked from the receiver for some period of time. After acquiring the signals, they may be maintained or “tracked.” Assuming a fixed sensitivity threshold, the time spent acquiring the SV signals is proportional to the total search space that is derived from the product of time and frequency uncertainty. For applications that desire high sensitivity, the signal re-acquisition delay may take tens of seconds if the search space is large.
In order to reduce this delay, information may be provided to aid a GPS receiver in acquiring a particular signal. Such assistance information permits a receiver to narrow the search space that must be searched in order to locate a signal, by providing bounds on the code and frequency dimensions. The predicted code window provides a reduced range within which the “code phase” (effectively, the signal time of arrival, or “pseudorange”) should be found, or a predicted range of observed Doppler shift associated with the signal. Assistance may also include other information about the signal, such as its PN (pseudo-noise or pseudo-random) code, data bit modulation, and content. Narrower code and frequency windows reduce the overall search space resulting in a reduction in the time in which the receiver takes to acquire the signal. A system that employs a GPS receiver augmented with externally sourced GPS assistance data is commonly referred to as an “assisted global positioning system” (AGPS).
One example of an AGPS system is a wireless mobile station (MS) with GPS capabilities in communication with one or more base stations (BSs), also referred to as base transmitting stations (BTSs) or node Bs, which in turn communicate with one or more servers, also called Position Determination Entities (PDEs) or Serving Mobile Location Centers (SMLCs) depending upon the communication air interface protocol. The PDE derives GPS assistance information from one or more GPS reference receivers. The PDE also has access to a means of determining the approximate MS position. This might consist of a “base station almanac” (BSA) that provides BTS/node B location based upon serving cell identification (ID) reported by the MS. Alternatively, this may be derived via a AnyTime Interrogation (ATI) request to the “home location registry” (HLR) associated with the MS. The PDE computes the assistance information customized for the approximate MS position. The BSA provides the approximate location of the MS based upon the serving cell identification provided to the PDE by the MS. The BSA provides the geographical coordinates for a reference position. The PDE also maintains a GPS database that contains reference time, satellite orbit almanac and ephemeris information, ionosphere information, and satellite working condition (“health”) information.
The goal of such GPS assistance information is to permit the MS to predict the time of arrival, or code phase, of a particular SV signal, and the Doppler shift of the SV signal. If the MS is provided with an initial reference position that is within an area of predefined size, such as a particular cellular coverage, then the total search space can be reduced to that consistent with the predefined size. Reducing search space size allows the receiver to spend more time processing each code and frequency hypothesis resulting in improved overall sensitivity. Sensitivity improvements in excess of 20 dB can be obtained by using reduced search space.
However, assisted position location systems depend upon communication with an external entity. Such communication suffers from connection and messaging latency, consumes additional power and consumes additional communication system bandwidth that impacts the overall capacity.
Position determination thus requires frequent updates of either or both orbital data or acquisition assistance for satellite signal acquisition. A need exists for a system and method that improves the performance and accuracy of position determination with diminishing dependence upon frequent updates of orbital data or satellite signal acquisition assistance.